1. Field of the Invention
The present invention relates to compound semiconductor wafers, and in particular to the profiling of carrier concentration in In-containing compound semiconductor wafers.
2. Description of the Background Art
Well-known methods of profiling carrier concentration within semiconductor wafers may be grossly divided into methods that measure Hall coefficient, and the C/V technique, which measures capacitance/voltage characteristics.
With the Hall-coefficient measuring methods, the carrier concentration of the semiconductor wafer cannot be measured nondestructively since a rectangular test piece for measuring must be cut out of a semiconductor wafer. This means that the carrier concentration of wafers themselves into which semiconductor devices are built cannot be profiled. Likewise, the Hall coefficient pertains to the test piece as a whole, and does not allow profiling of carrier concentration locally within the test piece.
In terms of the C/V technique, ordinarily a Schottky diode that is metal vapor-deposited onto the semiconductor wafer is formed, and an ac voltage of minute amplitude is superimposed onto a dc reverse-bias voltage to measure the C/V characteristics. Being that the region within the semiconductor wafer on which the Schottky diode for measuring C/V characteristics is formed cannot thereafter be employed for semiconductor device formation, the routine C/V technique cannot be said to be a non-invasive profiling method.
Likewise, with regard to compound semiconductor wafers, the routine C/V technique is not a very attractive method. The reason why is that in respect of compound semiconductor wafers, the barrier height of Schottky diodes is low, oxide-film formation cannot be controlled, and further, problems such as chemical reactions between metals and the compound semiconductor can arise.
Therein, to profile carrier concentration in a compound semiconductor wafer, electrochemical C/V techniques utilizing an electrolyte as an electrode have been used. (See for example, J. Electrochem. Soc., Vol. 133, 1986, pp. 2278–2283.)
Reference is made to FIG. 2, a block diagram schematically illustrating a conventional electrochemical C/V technique. In the electrochemical C/V analyzer set out in FIG. 2, the interior of a cell 1 is filled with an electrolyte 2 such as an aqueous HCl solution. A calomel electrode 3 as a reference electrode is inserted into the cell 1. The cell 1 has a ring-shaped opening 1a, and a compound semiconductor wafer 4 whose C/V characteristics are to be measured is contacted with the electrolyte 2 via the opening 1a, wherein the electrolyte 2 acts as one of the electrodes. A probe electrode 5 as the other electrode is contacted on the compound semiconductor wafer 4. An electrical analyzing unit 6 supplies a dc reverse-bias voltage and around a 3000-Hz ac superimposed voltage to the reference electrode 3 and the probe electrode 5 to measure the C/V characteristics.
In a depth w from the surface where the compound semiconductor wafer 4 is in contact with the electrolyte electrode 2, the carrier concentration N(cm−3) within the wafer may be determined using the following formula (1).N(w)=(−C3/qεA2)(dC/dV)−1  (1)
Herein, w expresses the depth from the wafer surface to the edge of the depletion layer. That is, N(w) expresses the carrier concentration N in the depth w from the wafer surface. Likewise, C expresses capacitance measured by a dc reverse-bias voltage; q, electronic charge; ε, permittivity; A, measurement area; and dC, variation in capacitance depending on variation dV in the superimposed ac voltage. Here, the depth w may be found from the following formula (2).w=∈A/C   (2)
In terms of an electrochemical C/V technique utilizing an electrolyte electrode as described above, profiling carrier concentration through depths of more than 3 μm is difficult unless a reverse-bias voltage that exceeds 10 V is applied. However, wherein an electrolyte electrode such as, e.g., an aqueous HCl solution is utilized, electrical breakdown of the electrolyte sets in when a high reverse-bias voltage in excess of 10 V is applied, giving rise to problems in that bubbles of hydrogen and oxygen cling to the wafer surface and make it impossible to measure the C/V characteristics. Applying too high a voltage can also give rise to problems in that the leakage current grows large, and electrolyte leakage occurs.
Consequently, in conventional electrochemical C/V analyzers, the maximum value of the reverse-bias applied voltage is in general limited to 10 V. To work around this limitation, carrier concentration through depths of more than 3 μm is profiled by repeating C/V analysis using an applied voltage of under 10 V, and wafer surface etching using a photo-etching process.
This means that, as shown in FIG. 2, the cell 1 in a conventional electrochemical C/V analyzer is furnished with a light-receiving window 1b. By shining light 7 onto the wafer surface where it contacts the electrolyte 2 at the ring-shaped opening 1a, the electrolyte 2 works as an etchant and the wafer surface is removed to a predetermined depth by photo-etching. Then a succeeding C/V analysis is performed with the surface freshly formed by the etching as a new reference.
With this method of profiling carrier concentration by repeating C/V analysis and photo-etching in this way, time is required for the etching; for a C/V analysis in order to profile carrier-concentration/distribution about 2 μm in the depth direction, around 1 hour is necessary. Furthermore, being that the photo-etched region cannot thereafter be employed for semiconductor device formation, the conventional electrochemical C/V technique cannot be said to be a non-invasive profiling method.
Moreover, In-containing compound semiconductor wafers for forming optical-communications photo detectors must be furnished with an epitaxial layer 5 to 8 μm in thickness, and if the carrier concentration within an epitaxial layer that thick is profiled by the repeating of C/V analysis and photo-etching processes, the C/V analysis alone ends up taking around 3 to 4 hours. Still further, performing photo-etching with consistency is not easy, and gaining high precision in C/V analysis is difficult.